Conventionally, as shown in FIG. 7, this type of robot is disposed in front of a working machine 50 such a press brake. The robot is constructed such that a first arm 53 which is rotatable on a horizontal axis (a first axis J1) is radially connected to an upper portion of a support strut 51 mounted on a floor by means of a first rotated joint 52. A second arm 55 which is rotatable on a horizontal axis (a second axis J2) is radially connected to a distal end of the first arm 53 by means of a second rotated joint 54. A third arm 57 which is rotatable on a horizontal axis (a third axis J3) is connected to a distal end of the second arm 55 in a coaxial direction by means of a third rotated joint 56. A hand 59 which is rotatable on a horizontal axis (a fourth axis J4) is connected to a distal end of the third arm 55 by means of a fourth rotated joint 58. These rotated joints are respectively provided with servo motors M1,M2,M3,M4 and position detecting means such as encoders for detecting the rotating positions of these rotated joints. By constantly detecting the position of the hand 59 with the position detecting means, the rotating angles of these rotated joints are controlled such that the hand 59 traces a plurality of points which are preliminarily taught and stored in memory. The coordinates of the position of the center of the hand 59 is Cartesian coordinates made of an X coordinate (a direction of arrow X) which is a traverse distance of the hand 59 when the hand 59 is moved in parallel in a horizontal direction by a combination of the rotations of the first to third rotated joints and a Y coordinate (a direction of Y) which is a traverse distance of the hand 59 when the hand 59 is parallelly moved in a vertical direction by a combination of the rotations of the first to third rotated joints. The posture of the hand 59 is controlled by two polar coordinates made of an A coordinate which is a rotating angle of the hand 59 around the fourth axis J4 and a B coordinate which is a rotating angle of the hand 59 around the third axis J3. The origin of the coordinates of four axes is arbitrary and a basic posture of the robot may be predetermined as such an origin. The hand 59 is provided with a gripping element such as a vacuum cup 60. The servo motors M1 to M4 are controlled by a microcomputer installed in a control panel 61.
With such a robot 62, a series of operations which comprise descending the hand 59 to suck and pick up a work which is not yet subjected any working from a work supply base 63, advancing and inserting one edge of the work into the working machine 50 to bend the work, retracting the work from the working machine 50 to a predetermined position where the hand is rotated about a horizontal axis, subjecting the other edge of the work to the working of the working machine 50, and stacking the work on a product stacking base 64 after working on the work is completed, and returning the hand 59 to the work supply base 63 to pick up a next work which is not yet subjected to any working, are carried out as a cycle and these cyclic operations are repeated. It must be noted, however, that the lifting amount of the hand is changed corresponding to the height of works remaining on the work supply base 63 as well as on the product stacking base 64.
To carry out the above-mentioned repetitious operations in a most efficient manner, the robot 62 is preliminarily taught so as to execute operations on the above-mentioned four axes in an overlapping manner or simultaneously. Since the robot 62 carries out the operations in accordance with sequences and timings which are preliminarily taught, no trouble occurs so long as the operations are carried out in a normal routine. However, when a trouble such as dropping of the work in the midst of the routine work occurs and the operation of the robot is stopped accordingly, the robot 62 is returned to a fixed position where the position of the work can be determined, e.g. a position on the work supply base or a position on the mold of the working machine and the work is gripped again. Then, the work is returned to a position where the robot is stopped while being gripped by the robot and the robot is operated again in accordance with a program of the original routine. Conventionally, in such a case, a next target position to which the hand is to be moved is taught and the hand is moved to the target position with an automatic manipulation or the hand is moved to the target position with a manual manipulation such that the hand is moved little by little by manually operating the axes one after another.
To move the hand to the taught target position with the automatic manipulation, however, the operations on the four axes are carried out simultaneously without any sequence, the work or the hand may heavily comes into contact with objects located around the robot and accordingly the hand may be broken or ruptured. On the other hand, the manual manipulation which is carried out by operating the axes one after another at a low speed takes a considerable operating time thus lowering an operating efficiency.
Accordingly, it is an object of the present invention to provide an interference preventing method which can enhance the efficiency in an operation to return the robot to the normal operation after the occurrence of a trouble and the robot which can carry out the method.